BUG/MINOR: resolvers: fix leaked dgram and dns_ring struct in parse_resolve_conf()

BUG/MINOR: resolvers: fix leaked dgram and dns_ring struct in parse_resolve_conf()

Some strdup() failures in parse_resolve_conf() do not release everything
due to the way the function is built, resulting in leaks on error that are
caught by ASAN:

  Direct leak of 304 byte(s) in 1 object(s) allocated from:
      #0 0x7fe74231ea23 in calloc (/usr/lib64/libasan.so.8+0x11ea23)
      #1 0x000000af1681 in dns_dgram_init src/dns.c:468
      #2 0x00000091cbbf in parse_resolve_conf src/resolvers.c:3559
      #3 0x00000092179e in cfg_parse_resolvers src/resolvers.c:3815
      #4 0x00000089d33e in parse_cfg src/cfgparse.c:2202
      #5 0x0000009e0a39 in read_cfg src/haproxy.c:1142
      #6 0x000000447e8c in main src/haproxy.c:3474
      #7 0x7fe74162ad13 in __libc_start_call_main (/lib64/libc.so.6+0x2ad13)
      #8 0x7ffc0a43e31f  ([stack]+0x2031f)

  Indirect leak of 131080 byte(s) in 1 object(s) allocated from:
      #0 0x7fe74231f0ab in malloc (/usr/lib64/libasan.so.8+0x11f0ab)
      #1 0x000000c73e19 in dns_ring_new src/dns_ring.c:59
      #2 0x000000af1848 in dns_dgram_init src/dns.c:480
      #3 0x00000091cbbf in parse_resolve_conf src/resolvers.c:3559
      #4 0x00000092179e in cfg_parse_resolvers src/resolvers.c:3815
      #5 0x00000089d33e in parse_cfg src/cfgparse.c:2202
      #6 0x0000009e0a39 in read_cfg src/haproxy.c:1142
      #7 0x000000447e8c in main src/haproxy.c:3474
      #8 0x7fe74162ad13 in __libc_start_call_main (/lib64/libc.so.6+0x2ad13)
      #9 0x7ffc0a43e31f  ([stack]+0x2031f)

  Indirect leak of 64 byte(s) in 1 object(s) allocated from:
      #0 0x7fe74231f0ab in malloc (/usr/lib64/libasan.so.8+0x11f0ab)
      #1 0x000000c73e09 in dns_ring_new src/dns_ring.c:55
      #2 0x000000af1848 in dns_dgram_init src/dns.c:480
      #3 0x00000091cbbf in parse_resolve_conf src/resolvers.c:3559
      #4 0x00000092179e in cfg_parse_resolvers src/resolvers.c:3815
      #5 0x00000089d33e in parse_cfg src/cfgparse.c:2202
      #6 0x0000009e0a39 in read_cfg src/haproxy.c:1142
      #7 0x000000447e8c in main src/haproxy.c:3474
      #8 0x7fe74162ad13 in __libc_start_call_main (/lib64/libc.so.6+0x2ad13)
      #9 0x7ffc0a43e31f  ([stack]+0x2031f)

  SUMMARY: AddressSanitizer: 131448 byte(s) leaked in 3 allocation(s).

Let's free the dgram and the dns ring. This can be backported though it's
not important as it only happens on OOM condition during boot.